1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (hereinafter also simply referred to as “photoconductor”), in particular, to an electrophotographic photoconductor that is used in copiers, facsimile machines, printers, or the like electrophotographic apparatuses.
2. Description of the Related Art
Image formation using an electrophotographic system is diversely applied to copiers, printers, plotters and digital imaging complex machines combining functions of these machines in the office, and recently also to small-sized printers and facsimile machines for personal use. Many types of photoconductors for these electrophotographic apparatuses have been developed since the invention by C. F. Carlson, see U.S. Pat. No. 2,297,691. Photoconductors today generally use organic material.
There is a type of organic photoconductor using organic material which is a functionally separated photoconductor and which consists of an undercoat layer, a charge generation layer, a charge transport layer, and a protective layer that are laminated on a conductive substrate. The conductive substrate is made of aluminum or the like. The undercoat layer can be an anodic oxide film or a resin film. The charge generation layer contains an organic pigment exhibiting the photoconductive property such as phthalocyanines or azo pigments. The charge transport layer contains a molecule having a partial structure that involves hopping conduction of charges, such as a molecule of amine or hydrazone that bonds with conjugated π electrons. Another type of known photoconductor, a single layer type photoconductor, comprises a photosensitive layer exhibiting both charge generating and charge transporting functions and a protective layer that are laminated on an undercoat layer.
Each layer is normally formed, because of mass-production, by dipping and coating a conductive substrate in a coating liquid prepared by dissolving or dispersing a pigment, a charge generation agent, to exhibit a charge generation or light scattering function, and a charge transport agent to exhibit a charge transport function.
In a so-called reverse development process that is primarily employed in recent electrophotographic apparatuses, an exposure light source uses a semiconductor laser or a light emitting diode with an oscillation wave length ranging from 450 nm to 780 nm; digital signals of a picture or characters are transformed into optical signals; the light is irradiated on an electrified photoconductor to form a latent electrostatic image on the photoconductor surface; and the latent image in turn is made visible by toner.
Among charge generation agents, phthalocyanines have been extensively studied as a material for a photosensitive layer because the phthalocyanines have a larger light absorbing capability in the oscillation wavelength region of semiconductor lasers than other charge generation agents and thus exhibit excellent charge generation ability. Known photoconductors use a variety of phthalocyanines having a central atom of copper, aluminum, indium, vanadium, or titanium.
There are two methods for electrifying a photoconductor: a non-contact electrification method by means of corona discharge from a scorotron in which the electrifying member is not in contact with the photoconductor, and a contact electrification method by means of a roller of conductive rubber or a brush of conductive fibers in which the electrifying member is in contact with the photoconductor. The contact electrification method has advantages, compared with the non-contact electrification method, including less generation of ozone because of a shorter discharge distance in the air, lower supply voltage, maintenance-free by virtue of no deposition of contamination on the electrifying member due to discharge, and a homogeneous electrification potential on the photoconductor. These advantages can achieve an electrophotographic device that is compact, low in price, and low in environmental pollution. Therefore, the contact electrification method is the mainstream method in medium to small-sized devices.
In a reverse development process, dark potential corresponds to a white field on an image, and bright potential corresponds to a black field. If the surface of the conductive substrate has structural defects such as significant irregularities or defects involving in inhomogeneity of material such as precipitation of impurities, these defects emerge as image defects such as black spots or fogging in the white field. These image defects can be considered to occur through local drop of the electrified potential at the location of the defects on the conductive substrate at which charge injection takes place into the photosensitive layer from the substrate due to the defects on the substrate. This tendency is particularly significant in electrophotographic devices employing both the reverse development system and the contact electrification system because of direct contact between the photoconductor and the electrifying member.
To address this problem in electrophotographic devices employing contact electrification system, an undercoat layer is generally provided between the conductive substrate and the photosensitive layer. The undercoat layer is composed of, for example, an anodic oxide film of aluminum, a boehmite film, or a resin film of poly(vinyl alcohol), casein, poly(vinyl pyrrolidone), poly(acrylic acid), gelatin, polyurethane, or polyamide. The resin film can contain particles of metal oxide such as titanium oxide or zinc oxide for the purpose of suppressing excessive reflection of exposure light from the substrate and avoiding a poor image due to interference fringes, and for appropriately adjusting the resistivity of the undercoat layer. The anodic oxide film, in particular, is known to give excellent stability of electrical potential under an environment of high temperature and high humidity, as disclosed in Japanese Unexamined Patent Publication No. H5-34964. A copolymerized nylon film is also widely used for an undercoat layer because it can provide a uniform thickness by means of dip coating and exhibits desirable mass-production and low price. International Patent Publication No. WO 85/00437 discloses a photoconductor for rear surface exposure using caprolactam as a component monomer for copolymerized nylon resin.
An undercoat layer currently in use has the problem that the electric properties change remarkably in the environment of operation, especially in an environment of high temperature and high humidity, and the electric resistivity changes due to moisture absorption in the undercoat layer causing fogging in the image. To cope with this problem, Japanese Unexamined Patent Publication No. S63-298251, for example, discloses use of an intermediate layer including a resin layer containing titanium oxide for the purpose of suppressing the environmental dependence. This document, however, only discloses an embodiment using a nylon resin having a special structure. Japanese Unexamined Patent Publication No. 2003-287914, discloses use of an intermediate layer including a polyamide resin of special structure to improve resistance to moisture. The document, however, fails to disclose an aromatic ring in the dicarboxylic structure in the component monomer and does not mention an effect from adding a monomer of aromatic dicarboxylic acid.
There is a further cause of image defects including black dots and fogging in a white field, that is, aggregation of metal oxide used in the undercoat layer. The aggregate, when it exists in the coating liquid, is also included in the film in the process of application and becomes a passageway for charges causing microscopic leak of charges towards the surface of the photosensitive layer. Thus, poor images result similar to image faults due to defects on the substrate.
Of the aggregates, coarse primary particles can be removed rather readily from coating liquid by a process of filtration, for example, while secondary particles, being formed by a relatively weak force of aggregation, cannot be removed. Therefore, it is important to avoid the formation of secondary particles by finding a composition that inhibits generation of such particles, by improving dispersion capability of the metal oxide, and by establishing an interaction with the resin to maintain a stable dispersion.